1. Field of the Invention
The invention generally relates to data communications devices such as WLAN (Wireless Local Area Network) transmitters and corresponding methods, and particularly to front end techniques in such devices.
2. Description of the Related Art
A Wireless Local Area Network is a flexible data communications system implemented as an extension to or as an alternative for a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wideband radio frequency technique developed for use in reliable and secure communications systems. The spread spectrum technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing Wireless Local Area Networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. Further extensions exist.
Examples of these extensions are the IEEE 802.11a, 802.11b and 802.11g standards. The 802.11a specification applies to wireless ATM (Asynchronous Transfer Mode) systems and is primarily used in access hubs. 802.11a operates at radio frequencies between 5 GHz and 6 GHz. It uses a modulation scheme known as Orthogonal Frequency Division Multiplexing (OFDM) that makes possible data speeds as high as 54 Mbps, but most commonly communications take place at 6 Mbps, 12 Mbps or 24 Mbps. The 802.11b standard uses a modulation method known as Complementary Code Keying (CCK) which allows high data rates and is less susceptible to multi-path propagation interference. Occasionally, the CCK modulation scheme is also referred to as DSSS-CCK (Direct Sequence Spread Spectrum CCK) modulation. The 802.11g standard can use data rates of up to 54 Mbps in the 2.4 GHz frequency band using OFDM. Since both 802.11g and 802.11b operate in the 2.4 GHz frequency band, they are completely interoperable. The 802.11g standard defines CCK-OFDM as an optional transmit mode that combines the access modes of 802.11a and 802.11b and which can support transmission rates of up to 22 Mbps.
WLAN transmitters and other data communications devices usually have a system unit that processes radio frequency (RF) signals. This unit is usually called front end. Basically, a front end comprises radio frequency filters, intermediate frequency (IF) filters, multiplexers, modulators, amplifiers, and other circuits that could provide such functions as amplification, filtering, conversion and more. Referring to FIG. 1, the front end usually includes a digital front end 100 which is the digital portion of a circuit which precedes digital-to-analog conversion. Thus, the digital front end 100 performs some digital signal processing and then outputs the digital signal to a digital-to-analog converter 110. The converted, i.e., analog, output signal of the digital-to-analog converter 110 is then supplied to an analog front end 120.
As can be seen from FIG. 1, the analog front end 120 of conventional data communications transmitters may have an analog signal processing unit 130 for, e.g., filtering or amplifying the analog signal received from the digital-to-analog converter 110. Then, a unit 140 may upconvert the signal output by the analog signal processing unit 130. Conventionally, baseband carriers conveying data by way of some modulation technique are upconverted from baseband to some other intermediate frequency through a process called mixing. Following the mixing process, the IF signal is further upconverted to an RF frequency in the desired transmission frequency band.
Transmitter architectures exist where unit 140 has zero-IF and/or low-IF topology. This will now be explained in more detail with reference to FIGS. 2 and 3.
FIG. 2 is a simplified diagram illustrating the zero-IF approach for integrated transmitters. In the zero-IF approach, the incoming signal which is at baseband (BB) frequency, is converted by mixer 200 directly to the transmission RF frequency. Such direct conversion architectures have simplified filter requirements and can be integrated in a standard silicon process, making this design potentially attractive for wireless applications. However, there may be problems with the DC offset, I/Q mismatch, and with low frequency noise.
FIG. 3 illustrates the low-IF approach. As can be seen, the low-IF architecture operates at an intermediate frequency close to the baseband (like the zero-IF approach) and can therefore be integrated like the zero-IF circuits. However, there are two upconverters 300 and 310 to convert the baseband frequency signals to intermediate frequency and then from intermediate frequency to the transmission RF frequency. Low-IF devices can avoid the problems of DC offset, I/Q mismatch and low-frequency noise, but may require additional LO-feedthrough cancellation. For this reason, an LO-feedthrough cancellation unit 320 is added in the low-IF topology.
Thus, the zero-IF and low-IF approaches each have their own advantages and disadvantages. This is why conventional communications devices exist that use either the zero-IF approach or the low-IF approach in the analog front end. Further, dual band RF transceivers for WLAN systems exist where a direct conversion technique is used for one WLAN mode and a low-IF architecture is used for another WLAN mode.